Whisk flour, salt and baking powder in a small bowl; set aside. Spray an 8-inch baking pan with cooking spray. Fit a sheet of foil in pan and up two sides, so you can use it as a handle to pull cooked brownies from pan (I used parchment paper). Spray sheet with cooking spray.
Melt chocolates and butter in a medium bowl over a pan of simmering water. Remove from heat; whisk in sugar and vanilla. Whisk in eggs, one at a time, fully incorporating each one before adding the next. Continue to whisk until mixture is completely smooth and glossy. Add dry ingredients, whisk until just incorporated.
Pour batter into prepared pan; bake until a toothpick or cake tester inserted in center comes out with wet crumbs, 25-30 minutes.
When the brownies have about 5 minutes left to bake, make the peanut butter filling.
Peanut Butter Filling
Place the peanut butter chips and the butter into a microwaveable glass bowl. Cook at medium power for 1 minute. Stir and continue to cook on medium power in 30 second intervals until mixture is melted and glossy in appearance.
When the brownies come out of the oven, pour peanut butter mixture over the top of the warm brownies. Use a heat resistant spatula to spread the mixture evenly over the top. Place the brownie pan on a wire cooling rack to cool completely.
While brownies are cooling, make the ganache and allow it to chill in the refrigerator. Once it has chilled completely, place it into the bowl of a stand mixer fitted with the whip attachment and whip the ganache until it is light, fluffy, and spreadable (about 5-8 minutes).
For the Ganache
Put chocolate into a heatproof bowl.
In a medium saucepan, bring cream to a simmer (do not boil). Pour cream over chocolate. Let stand for 5 minutes, then whisk until smooth. Transfer to a heat proof container and place in refrigerator for 15 minutes or until mixture is completely chilled, whisking occasionally.
After ganache has chilled, transfer to the bowl of a stand mixer and whip on high speed for 2-3 minutes, or until light and fluffy. Use a frosting spatula or knife to apply ganache to the tops of brownies. Garnsih with mini chocolate chips if desired. Cut into squares and serve.
Store well covered at room temperature for 2 days, or in a refrigerator for up to a week.